I notice the "1.25" in your description, which would indicate a 1.25 GHz iMac, correct?
That model will certainly upgrade to 2GB RAM, 2 x 1GB, and each 1GB is a different type. PC-2700 DDR SDRAM. There's a SO-DIMM slot under the metal bottom plate. The second slot is a full-size SDRAM (PC-2700 DDR again), and you have to take off the bottom case to get to that one. AND, the heat sink has to be cleaned and new compound spread when re-assembling the case. The take-apart vids should take you through that very nicely.
While you have the bottom off - that's your opportunity to replace the hard drive, too. Those iFixit vids should show you how to do that, too. Correct tools are important on that iMac. Makes the job go a lot easier.
Takes a PATA hard drive, which is getting harder to find.
There MIGHT be room for an SATA drive, with an SATA-to-IDE adapter, but I've never tried that one, it's a pretty close fit for the hard drive. SATA would make finding a hard drive a lot easier.
The G4 iMac is still my favorite.